There are several ways in which two boats can become overlapped on the same tack within two boatlengths. One boat could overtake another either to windward or to leeward. A boat could tack or gybe next to another boat. Two boats overlapped more than two boatlengths apart sailing different courses could converge.

If a boat that was clear astern, overtakes another boat and becomes overlapped to leeward of her, then the leeward boat may not sail above her proper course (see rule 17 for full details). This rule exists so that:

1. If a boat overtakes you to windward, you can luff her up to protect yourself from getting rolled, and
2. If a boat overtakes you to leeward, she cannot luff you up in order to pass.

Does that help clear it up?

Regards,
Eric